Certain microfluidic devices are commercially available. Such devices may be used to interrogate particles carried in a fluid diluent. Exemplary particle interrogation may be electrically-based, optically-based, or both. One effective electrically-based interrogation encompasses particle analysis evidenced by the Coulter effect. An operable optically-based interrogation encompasses detection of fluorescence conventionally known as a Stokes' shift.
Operable microfluidic devices are conventionally arranged to accept a fluid sample that generally encompasses a subset of a bulk fluid. One desirable interrogated sample size for interrogation of whole blood is about 50 μl. An operable arrangement entails loading a 75 μl super-sample into a microfluidic interrogation cassette, of which only 50 μl is typically interrogated. The extra 25 μl may be used to ensure operational readiness of the cassette. Sometimes, essentially the entire super-sample may be interrogated.
A super-sample may be extracted from bulk fluid using a pipetter, syringe, or other fluid transfer device, that can accurately dispense a small quantity of fluid into a receiving well, or entry aperture, in a microfluidic interrogation device. Conceptually, a vent may permit escape of air from the microfluidic interrogation device as the super-sample is injected into the device. In certain cases, surface tension forces may be sufficient to draw the super-sample into a receiving area of the channel. In other cases, a vacuum may sometimes be applied from an external source to the vent to reduce the pressure in a channel passing through the microfluidic device to assist in loading a fluid super-sample into a receiving portion of the channel. In any case, a vacuum is conventionally applied to cause an interrogated-sample portion of fluid to flow through the microfluidic device while performing particle analysis.